1. Field of the Invention
The present invention relates to an image shake correction technique for correcting an image shake caused by shake, such as hand shake. In particular, the present invention relates to a technique for smoothly correcting the low frequency component of a shake while ensuring stable performance immediately after the power is turned on.
2. Description of the Related Art
In cameras having an image shake correcting apparatus for preventing image shake caused by hand shake, or the like, mounted therein, shooting can be performed without image shake even when hand shake occurs upon shutter release. An angle shake of a camera caused by hand shake, or the like, is detected and, then, an image shake correction lens (hereafter referred to as a “correction lens”) is driven, depending on a detection value. At this time, camera vibration needs to be correctly detected so as to correct an optical axis change caused by shake. In principle, a vibration detecting unit configured to obtain the results of detection, such as an angular velocity and a drive control unit configured to displace a correction lens based on the results of calculation processing are mounted in a camera so as to suppress image shake.
The output signal of a sensor for detecting camera vibration includes a direct current component, such as variations in reference voltage due to individual differences in sensors and drifts in accordance with a change in temperature. If the sensor output includes low frequency component noise, such low frequency component noise may lead to deterioration in correction precision. Thus, in order to remove an offset component, a low frequency component is typically removed from the output signal of a sensor using an HPF (high-pass filter) so as to obtain an image shake correction signal.
When a user performs an operation, such as panning or tilting, for moving an imaging apparatus in one direction for a relatively long period of time, the output signal of a sensor includes a large amount of low frequency components, and thus, low frequency components need to be attenuated upon image shake correction. There has been known a control for determining whether panning or tilting is being performed, based on data of angular velocity and data of an angle obtained by integration of angular velocity so as to perform switching to a correction characteristic suitable for panning or tilting. Control is made such that image shake correction does not respond to a low frequency by transitioning the cut-off frequency of an HPF or an integrating filter to a higher frequency side. Japanese Patent Laid-Open No. H5-323436 discloses image shake correction control in the panning state or the tilting state, which is capable of performing image shake correction for high frequencies, while suppressing the response at low frequencies. Japanese Patent Laid-Open No. H10-010596 discloses anti-shake control by fixing an offset during imaging without performing HPF processing during imaging (exposure).
In the conventional configuration for attenuating a low frequency component using an HPF for removing an offset component, the following phenomenon occurs. If the output of an angular velocity sensor includes a low frequency noise component upon image shake correction based on the output of the angular velocity sensor, unsuitable correction is made to actual camera shake. In addition, image shake correction may be adversely affected by a filter characteristic including a secondary HPF due to panning, or the like. The low frequency component of large amplitude is attenuated by the occurrence of vibration caused by panning, or the like, and a signal in a direction reverse to the panning direction is generated, for example, upon completion of panning (a so-called “swing-back phenomenon”). The signal is then slowly converged to zero. However, if image shake correction is performed based on the signal, the correction amount is calculated by a signal that is different from the actual shake of an imaging apparatus, which may lead to deterioration in correction precision.
If the cut-off frequency of the HPF is set to low in the filter configuration, including the HPF, the performance of image shake correction for low frequency components associated with vibration, or the like, of the photographer's body can be improved. In this case, the magnitude of a swing-back signal becomes large, and the time to be taken until the signal is converged to zero becomes longer after the occurrence of large vibration caused by panning, or the like. Thus, an appropriate image shake correction effect may be obtained only when the photographer captures an image with his camera firmly held by his hands, so as not to excite shake of the imaging apparatus.
In the technique disclosed in Japanese Patent Laid-Open No. H10-010596, correction is performed in a state different from the actual hand shake when an angular velocity offset is fixed immediately prior to imaging, and an error occurs in the fixed angular velocity offset. In other words, when an offset error occurs in angular velocity, immediately prior to imaging, image shake correction is executed in a state when the angular velocity of the offset error is continuously added to a hand shake component signal during imaging. Hence, image shake correction is performed in an unintended direction, resulting in a reduction in correction effect.
Thus, it is preferable that no HPF is provided in order to improve an image shake correction effect in a low frequency range. However, in this case, since there exist individual differences among sensors for angular velocity detection, it is contemplated that a temperature drift, in which its direct current component changes due to variations in reference voltage, or the like, may adversely have an affect thereon. If a large angular velocity offset occurs when the power is turned on, a long time is required until the filter becomes stable. Degradation in the image shake correction performance is concerned for a long time, immediately after the power to the sensor is turned on. Thus, a target value needs to be calculated by subtracting an offset value from the output of the sensor when the power is turned on, but it is difficult to instantly calculate an offset value for the sensor immediately after the power is turned on.